Problems of 15v15 battles:
As pub players lack coordination, people tend to care about their own stats rather than victory, as every individual only have a small influence on the battle outcome. Result: selling teammates, pushing teammates, sniping in the back.
Maps are too small so every line is well defended, leaving little space for breakthrough and flanking. Result: camping meta.
15 ppl means at least 5 sniping TDs on each side, especially in tier 8. If you make a too aggressive move you get sniped dead within seconds. Result: camping and hull down meta.
If teams are smaller battles would be more fun and more fast paced, discouraging the usage of arties.
